This invention relates to the field of incinerators. In particular, the invention provides a hearth structure which lasts longer and requires considerably less maintenance than the incinerator hearths of the prior art.
A major problem with incinerators is the formation of slags. When certain waste materials are burned at high temperatures, the non-combustible products include both ash and slag formers. As used herein, the term "slag" refers to a product which takes the form of a solid mass, in contrast to ash which has a more powdery texture. Glass and metals tend to produce slags and, when molten, they tend to dissolve ash material and wet the refractory lining of the incinerator. Upon cooling, they solidify into a hardened mass.
The slag formed from glass and metals burned in an incinerator tends to adhere tightly to the refractory combustion surface of the hearth. After repeated use, wherein the incinerator has undergone many cycles of heating and cooling, a large quantity of slag will have accumulated on the combustion surface. Eventually, the pile of slag becomes so large that it seriously degrades the performance of the incinerator, by occupying space that should be occupied by fresh waste, by absorbing heat that is intended to be transmitted to the waste itself, and also by obstructing the combustion surface and thereby making it more difficult to remove the non-combustible products.
As noted above, slag adheres tightly to the refractory lining of the hearth. This adherence results from the fact that slag is a good solvent for refractory materials. Thus, it is very difficult to remove the slag without also removing part of the refractory lining. In typical conventional incinerators, one uses a "plow" or piston that periodically pushes the combustion products along the bottom of the hearth and out of the hearth. If slag has adhered to the bottom of the hearth, the plow may not be able to remove all of the slag. Moreover, the slag that is removed will carry away pieces of the refractory lining.
When an unacceptably large amount of slag has accumulated, the only known way to remove it has been to deactivate the incinerator until it is cool enough to allow workers to enter the hearth. The workers then remove the slag manually, by chipping it away. This procedure is extremely costly, not so much because of the labor required, but mainly because of the length of time during which the incinerator must be idled. Because the incinerator operates at very high temperatures, it requires a long time to cool. The bulk temperature inside the hearth is typically about 1400.degree.-1900.degree. F., and the temperature at the center of combustion can be in the range of about 2200.degree.-3000.degree. F. Thus, it is usually necessary to wait two or three days before the hearth is cool enough for workers to enter. Meanwhile, the user must find alternative means for disposing of waste. The losses due to "down time" can therefore be very large.
Eventually, removal of slag from the bottom of the hearth will damage the refractory lining so much that the lining must be replaced. Replacement of the lining is expensive, not only because of the labor and materials needed, but also because of the "down time" of the incinerator.
The problems described above are particularly acute when the waste being incinerated contains a large amount of glass. Molten glass is especially likely to form slag. While it is possible to prevent the molten glass from adhering to the hearth bottom by reducing the temperature of the hearth, this approach is not satisfactory in general, because the lower the temperature, the less efficient the combustion. If the temperature of the hearth is too low, then materials such as plastics, among the waste products intended to be incinerated, may solidify on the bottom of the hearth, instead of being burned.
The present invention provides an incinerator which solves the problems described above. In the incinerator of the present invention, slag is less likely to accumulate, and therefore the "down time" of the unit is minimized, while the useful life of the refractory lining is prolonged.